True vertical depth (TVD) of a vertical or horizontal well is calculated using the measured depth, inclination, and azimuth values logged at a series of survey points along the well. Example calculation methods include the tangential, average tangential, balanced tangential, mercury, radius of curvature, and minimum curvature methods. The delta (change in) TVD is calculated between each pair of survey points. The TVD of the well is the summation of all delta TVD calculations for each survey point, taking into account distance between sensor and bottom of the hole.